1. Field of the Invention
The present invention relates to an optical disk driving apparatus that records, reproduces, and erases information stored on an optical information medium, for example, an optical disk or an optical card, an optical disk system, a vehicle equipped with the optical disk system, a method of correcting spherical aberration in the optical disk driving apparatus, a program, and a recording medium.
2. Related Art of the Invention
Optical memory techniques using an optical disk with a bit-like pattern as a high-density, high-capacity storage medium have been expanding in application and put to practical use for digital audio disks, video disks, text file disks, and data files. Functions of utilizing a fine focused light beam to reliably and accurately record and reproduce information on and from the optical disk are roughly classified into a light converging function, focus control and tracking control provided by an optical system, and pit signal (information reproduction signal) detection.
In recent years, to further increase the recording density of the optical disk, efforts have been made to increase the numerical aperture (NA) of an objective lens that focuses the light beam on the optical disk to form a fine spot corresponding to a diffraction limit.
Furthermore, to reduce costs of the optical disk system, attempts have been made to form the objective lens using resin.
A major disadvantage of a resin objective lens with a high NA is that the refractive index of the lens varies with temperature. Variation in refractive index means a deviation in the refractive power of a lens surface from a design value. This may cause spherical aberration. Aberration of a lower order more significantly degrades the quality of information reproduction signals. Third-order spherical aberration is particularly disadvantageous. Thus, efforts have been made to provide an optical disk driving apparatus using the resin objective lens with the high NA. By way of example, FIG. 15 shows contents disclosed in Japanese Patent Laid-Open No. 2007-328886.
In an optical head device shown in FIG. 15, divergent light emitted by a blue light optical system 51 with a blue light source passes through a beam splitter 161 and is changed to parallel light by a collimate lens 205. The light is then converged, by an objective lens 50, on an information recording surface of an optical disk 9 (third generation optical disk) with a base material thickness of 0.1 mm. The light reflected by the optical disk 9 follows the opposite path and is then detected by a detector in the blue light optical system 51.
Divergent light emitted by a red light optical system 52 with a red light source is changed to parallel light by the collimate lens 205. The light is then converged, by the objective lens 50, on an information recording surface of an optical disk 10 (second generation optical disk) with a base material thickness of 0.6 mm. The light reflected by the optical disk 10 follows the opposite path and is then detected by a detector in the red light optical system 52.
A configuration with an infrared light source is also disclosed by Japanese Patent Laid-Open No. 2007-328886.
To correct a change in spherical aberration in blue light on the optical disk 9 caused by a change in temperature, a change in the temperature of an optical pickup device or an optical element is measured by a temperature sensor 53 to allow the collimate lens 205 to be traveled in the direction of an optical axis.
In this configuration, the distance by which the collimate lens travels is determined based only on the temperature obtained from the temperature sensor. This is what is called open loop control. However, the thus determined travel distance of the collimate lens may involve an error. Major error factors include the accuracy of the temperature sensor (an error of several degrees), variation, among individual light sources, in the amount by which wavelength changes depending on temperature, and a deviation of the dependence of the wavelength change on temperature from linearity. Here, the deviation from the linearity corresponds to a deviation from a proportional relationship between the wavelength change amount and the temperature change amount. The deviation from the linearity is caused by mode hopping or the like.
A deviation of the third-order spherical aberration in the resin objective lens caused by the temperature change is about 1 mλrms per degree centigrade when the numerical aperture of the objective lens is 0.6.
On the other hand, when the numerical aperture of the spherical lens up to 0.85, the deviation of the third-order spherical aberration in the resin objective lens caused by the temperature change increases up to 3 mλrms to 10 mλrms, though the deviation also depends on a focal distance. Thus, on the assumption that an error in the temperature sensor is 2° C., an error in spherical aberration may be at least 6 mλrms. For proper signal reproduction, the error in spherical aberration is desirably kept equal to or less than 10 mλrms . Thus, an error of at least 6 mλrms caused by the single factor, that is, the temperature change, is intolerable. Furthermore, if the deviation of the third-order spherical aberration per degree centigrade exceeds 3 mλrms, the error further increases and is more intolerable. Thus, the above-described method of correcting the third-order spherical aberration is insufficient, in which the travel distance of the collimate lens is determined based only on the open loop control using the temperature obtained from the temperature sensor.
In view of the problems with the conventional method of correcting spherical aberration, an object of the present invention is to provide an optical disk driving apparatus that enables information signals to be properly reproduced in spite of the use of an objective lens mainly composed of resin, as well as a related optical disk system, a vehicle utilizing the optical disk system, a method of correcting spherical aberration in the optical disk driving apparatus, a program, and a recording medium.